This invention relates to lasers, and, more particularly, to a laser system having a passive Q-switch.
A laser is a device that emits a spatially coherent beam of light of a specific wavelength. In a laser, a lasing element is placed within a laser resonator cavity and pumped with an energy source such as a flash lamp. The pumping action produces stored energy and gain within the lasing element. When the gain exceeds the losses so that there is a net light amplification per round trip of the light in the resonator cavity, laser light begins to build up in the cavity, and stored energy is extracted from the lasing element. This energy can be released in the form of a very short, intense light pulse by using a device called a Q-switch.
A Q-switch operates by initially increasing the cavity losses, thus preventing lasing action, while an amount of stored energy and gain is achieved that greatly exceeds the losses that would otherwise exist. The Q-switch losses are then quickly lowered, producing a large net amplification in the cavity, and an extremely rapid buildup of laser light occurs. The light pulse begins to decay after the stored energy in the lasing element has been depleted such that the gain once again drops below the cavity losses.
The Q-switch can be an active device which is controlled or driven by an external signal. The Q-switch can also be a passive structure that has no external control, but instead operates periodically as a result of its own properties. The present invention relates to a laser system using such a passive Q-switch.
A saturable absorber can be used as a passive Q-switch. The saturable absorber is a crystal having transmittance properties that vary as a function of the intensity of the incident light that falls upon the crystal. When light of low intensity is incident upon the saturable absorber, its light transmittance is relatively low, resulting in high cavity losses. As the incident light energy increases due to the buildup of energy within the laser resonator cavity, the light transmittance of the crystal increases. At some point, the light transmittance increases to a level such that the crystal "bleaches", i.e., becomes transparent, so that the cavity losses become low, and an intense Q-switched light pulse is emitted.
A significant limitation on the performance of saturable absorber Q-switch materials is their susceptibility to damage resulting from passage therethrough of an intense light beam. As indicated, the passive Q-switch causes the buildup of intense light within the laser resonator cavity. When the Q-switch crystal bleaches and the light pulse passes therethrough, the crystal may be damaged by the intense light, so that the ability of the crystal to function as a saturable absorber for subsequent cycles is impaired.
The properties of a saturable absorber crystal depend upon the wavelength of the incident light. A crystal which performs as a saturable absorber at one wavelength typically will not perform in the same manner at significantly different wavelengths. Further, a crystal may act as a saturable absorber for relatively low incident intensities, but higher intensities may damage the crystal. There is therefore an ongoing search for effective saturable absorber crystals for use as Q-switches in particular wavelength ranges, and which are robust (i.e., have high damage thresholds).
One of the laser operating ranges of interest is at about 1.5 micrometers wavelength. This wavelength range is of particular interest because light in this range will not damage the human eye at moderate intensities. For example, the Er:glass laser emits light at about 1.53 micrometers wavelength, and is of interest as an eye-safe laser. (In this accepted notation, A:B indicates a material having an ion of A doped into a B host crystal.) In the past, Q-switching of the Er:glass laser has been accomplished by an active, rotating prism Q-switch. Passive Q-switches of the saturable absorber type, with a satisfactorily high damage threshold, are not known.
Saturable absorbers that can be used as passive Q-switches operating in the 1.5 micrometer wavelength range are also known. Examples include Er.sup.3+ :Ca.sub.5 (PO.sub.4).sub.3 F, U.sup.2+ CaF.sub.2, U.sup.2+ BaF.sub.2, U.sup.2+ :SrF.sub.2, and Er.sup.3+ :CaF.sub.2. However, these fluoride-based materials have been observed to have relatively low damage thresholds. They are thus operable as Q-switches in laser systems to produce only relatively low-intensity external laser beams.
There is a need for saturable absorbers operable in the 1.5 micrometer wavelength range, which are robust and resistant to damage from the passage therethrough of a high-intensity laser beam. The present invention fulfills this need, and further provides related advantages.